The present invention relates to fluid machines, and in particular the present invention relates to fluid motors or pumps which have a rotating cylinder block and which have improved fluid flow into and out of the cylinder block.
There is a known class of fluid machines called axial piston pumps or motors. These machines may be operated as pumps by applying a rotary input to the device and producing thereby a flow of fluid under pressure, or they may be operated as motors by supplying fluid under pressure and deriving a rotary mechanical movement. In the specification which follows and the claims which form a part thereof, the present invention will be described as a pump. It should be understood that the invention applies equally to fluid machines generally, both pumps and motors.
Axial piston pumps of the bent axis type include a housing in which there is a rotary cylinder block having a plurality of at least approximative parallel cylinder bores within which cylinders reciprocate. The pistons extend from the cylinder barrel and are connected with a thrust plate by means of articulated connecting rods. The thrust plate rotates about an axis which intersects and is transverse to the axis of rotation of the cylinder block.
The cylinder barrel has an end face which bears against a valve plate. The valve plate has an inlet port and an outlet port which serve in a well known manner to provide properly phased communication between the cylinders in the cylinder block and the inlet and outlet passages of the device. The surface of the cylinder barrel against which the valve plate abuts may be a polished flat surface, but it may also be a curved surface, especially a spherical surface, and it will be seen that the present invention is applicable equally to both.
Many pumps and motors of the type described have variable displacements, and when operated as pumps can produce a flow in either direction without changing the direction of the rotary mechanical input to the pump. The present invention, however, is suited for fixed-displacement pumps and motors, or at least to machines in which the top and bottom dead center positions of the pistons do not reverse themselves with any possible change of displacement.
Considerable attention has been paid to the flow of fluid into and out of the cylinders of axial piston pumps and motors with a view to reducing frictional losses and improving efficiency. For example, it is already known that because of the rotary movement of the cylinder block, the flow of fluid through the valve plate and into the cylinder block should not be perpendicular to the end face of the cylinder block, instead the flow should be at an angle to this surface. Similarly, the flow of fluid out of the cylinder block should be inclined with respect to the end face of the cylinder block when viewed from a fixed frame of reference. Because of the high speed of the flow, the fluid has kinetic energy associated with it, and this energy can be utilized to increase pump efficiency by the flow is properly guided in the inlet ports of the cylinder barrel and inlet port in an end face of the valve plate and vice versa in case of a motor.
Furthermore, it has been established that the angle of the flow toward or away from the end face of the cylinder block is dependent on the side to which the axis of rotation of the cylinder drum is inclined relative to the plane of rotation of the thrust plate. See, e.g., German Published Application (Auslegeschrift) No. 13 01 712. In the device disclosed therein the flow of fluid into or out of the cylinder block is guided by careful shaping of the passages through the valve plate. The valve plate includes two arcuate, nearly semicircular openings. Two passages lead through the valve plate from external fluid connections, one of the passages leading to each of the arcuate openings. The arcuate openings have a tapered depth when measured parallel to the axis of rotation of the cylinder block. The depth of the arcuate openings at any location is in proportion to the rate of fluid flow to or from the cylinder when the passage leading from the cylinder to the arcuate opening is at that location.
This arrangement of tapered arcuate ports in the valve plate is often difficult to carry out in actual application. In practice it is often required that the external connections to the pump or motor must be made on a surface of the housing which is generally perpendicular with the axis of rotation of the cylinder block. This means the fluid must flow in (or out) of the pump or motor in a direction generally perpendicular to the end surface of the cylinder block. The passage or channel leading fluid into the pump widens circumferentially and narrows radially as it reaches the bottom of the valve plate where it forms the arcuate opening. The external connection with this passage is in the direction of the middle of the opening and as a consequence of the fact that the external connections have been centered with respect to the arcuate opening with which they communicate, one part of the flow must move in a direction away from the original axis of flow and counter to the direction of rotation of the cylinder block, and another part of the flow must move in the opposite direction. But the cylinder block rotates in only one direction, and thus the part of the flow which moved counter to the direction of rotation of the cylinder block must turn approximately 180.degree. in order to enter the cylinder. Moreover, the transistion from a circular channel or passage where the external connections to the valve plate are made to an arcuate opening in the valve plate is so rapid that cavitation and turbulence as well as separation of the flow from the walls are likely to result. This can result in incomplete filling of the cylinder and therefore inefficiency in the pump or motor.
Other attempts have been made to improve the flow of fluid into axial piston machines, whether pumps or motors. U.S. Pat. No. 3,202,101 shows a fixed displacement, swash-plate pump in which the fluid flowing to the port plate is driven by an impeller connected with the cylinder barrel. The impeller has flat blades which extend radially outward from a hub coaxial with the axis of rotation of the cylinder block. The fluid is given a rotary component of motion equal to the speed of the cylinder block and then fed to one of the arcuate openings in the valve plate. This pump occupies a great deal of space in the axial direction, and for this reason the pump may not be suitable for some applications. Further this design does not account for the fact that the arcuate opening in the valve plate extends for less than 180.degree., nor for the fact that the rate of intake flow into the cylinders varies with the sinusoidal reciprocation of the pistons in the cylinders.
German Disclosure (Offenlegungsschrift) No. 16 53 417 shows an axial piston pump in which the inlet flow is radially inward through channels which rotate with the cylinder block. This gives the flow an appropriate circumferential component but increases the radius of the cylinder block and thus the overall size of the pump. Further, the added weight to the cylinder block and its radial distance from the axis of rotation of the cylinder block increases the rotational moment of inertia of the cylinder block which is a disadvantage when the speed at which the pump is driven varies.
Finally, U.S. Pat. No. 3,223,047 discloses an axial piston pump having a vane dividing the incoming fluid flow, but its purpose is not related to improving the flow characteristics of the incoming fluid but merely to provide a supply of fluid for cooling purposes when the pump is operated at partial capacity.